Hollow glass spheres are widely used in industry as additives to polymeric compounds, e.g., as modifiers, enhancers, rigidifiers and fillers. Generally, it is desirable that these spheres be strong to avoid being crushed or broken during further processing of the polymeric compound, such as by high pressure spraying, kneading, extrusion or injection molding. It also is desirable in many cases that these spheres have densities close to that of the polymeric compound into which they are introduced in order that they distribute evenly within the compound upon introduction and mixing. Furthermore, it is desirable that these spheres be resistant to leaching or other chemical interaction with their associated polymeric compound.
The method of expanding solid glass particles into hollow glass spheres by heating is well known. See, e.g., U.S. Pat. No. 3,365,315. Glass is ground to particulate form and then heated to cause the particles to become plastic and for gaseous material within the glass to act as a blowing agent to cause the particles to expand. During heating and expansion, the particles are maintained in a suspended state either by directing gas currents under them or allowing them to fall freely through a heating zone. Sulfur, or compounds of oxygen and sulfur, serves as the principal blowing agent.
A number of factors affect the density, size, strength, chemical durability and yield (the percentage by weight or volume of heated particles that become hollow) of hollow glass spheres. These factors include the chemical composition of the glass; the sizes of the particles fed into the furnace; the temperature and duration of heating the particles; and the chemical atmosphere (e.g., oxidizing or reducing) to which the particles are exposed during heating.
There have been problems in attempting to improve the quality and yield of hollow glass spheres. One reason is that it was believed that the percentage of silica (SiO.sub.2) in glass used to form hollow glass spheres should be between 65 and 85 percent by weight and that a weight percentage of SiO.sub.2 below 60 to 65 percent would drastically reduce the yield of the hollow spheres. Also, the prior art has overlooked the significant improvement in strength of hollow glass spheres achieved through the optimization of particle size.